This application is a 371 of PCT/EP00/10989 filed Nov. 7, 2000.
The production of chlorine is among the most widespread process in the world-wide scenery of industrial chemistry. The current annual production, which can be esteemed as about 50 million tons, comes almost entirely from the electrolysis of alkali chlorides in aqueous solutions; in these processes, chlorine is evolved through the anodic discharge of chloride ions, typically with the concurrent production of alkali at the cathodic compartment; in the most typical case at the cathode also the reaction of hydrogen evolution takes place. Of the three types of electrolytic cells most commonly employed for this purposexe2x80x94the mercury cathode, membrane and diaphragm onesxe2x80x94the latter still accounts for the highest global amount of chlorine produced in the world-wide market. FIG. 1 shows a modem diaphragm cell, comprising an anodic base (1), made of a copper body lined with a thin sheet of titanium, whereupon anodes: (2) are fixed by means of current collecting copper stems (4), also protected with a titanium coating. The reason for these bimetallic constructions arises from the fact that copper, employed for its excellent electric properties, would be easily corroded by the anolyte (chlorinated brine), toward which, on the contrary, titanium shows good resistance characteristics. The cathode (3), on one side of which, and precisely on the side facing the anode, a diaphragm is deposited, is made of foraminous iron sheets or meshes. The cover (5), made of a chlorine-resistant plastic material, is provided with an outlet duct for the gaseous product chlorine (6) and of an inlet duct for brine feeding (not shown). The hydrogen and the alkaline solution (e.g. caustic soda solution) produced at the cathode exit respectively from the ducts (7) and (8). The diaphragm, having the purpose of separating the anodic and cathodic compartments, was traditionally made of asbestos fibres and a plastic binder; the need of abandoning the use of asbestos, noxious for the health, together with the quest for higher yields and longer duration of the elements, led to a radical re-thinking of the traditional diaphragms in terms of materials. Present day diaphragms are typically constituted of zirconium oxide fibres, or of plastic materials. Whereas asbestos based diaphragm were the component which determined the lifetime of the whole cell (an average of 10-14 month), the availability of the diaphragms of the new generation, known as xe2x80x9cNADxe2x80x9d (non-asbestos diaphragm), would allow extending the operative time of a diaphragm cell from a minimum of 36 to a maximum of 60 months, before their degradation. The current experience, however, indicates that another factor limits the total lifetime of diaphragm electrolytic cells for the production of chlorine, and is substantially associated with corrosion phenomena in the anodic compartment. In particular, the seal between the bimetallic current collecting stem (4) whereupon the anodes (2) are secured, and the copper anodic base (1) is realised by means of a gasket (9), as shown in FIG. 2. The experience on the best currently available gaskets permits to forecast a lifetime of 12-24 months in the typical operating conditions. The multiplicity of seals in a cell, wherein several tens of anodes (typically 40 to 90) are present, further increases the probability for a gasket to undergo a rupture, or anyway that it give rise to a leakage, well before the lifetime of the NAD diaphragms is over. When a leakage occurs in correspondence of the anodic stems (4), it is necessary to shut-down the cell because the following phenomena, each one of which is critical, take place:
impairment of the bimetal of the anodic stem (4), due to the corrosive action of the electrolyte.
impairment of the copper base, due to the same phenomenon
risk of electric grounding of the cell.
On the other hand, the shut-down of the cell and its opening for replacing the gaskets implies also the need of replacing the diaphragms, which during operation undergo a permanent deformation hampering their use in a subsequent assemblage. Assuring a leak-free sealing of the anolyte towards the anodic current collecting stems for the maximum lifetime of the NAD diaphragm (60 month), is an issue of fundamental importance in the economics of the diaphragm chlor-alkali electrolysers, as it would not be acceptable to nullify even partially the improvements that NAD technology has introduced on the duration of the diaphragms. FIG. 2 represents the state of the art in the field of sealing of the anodic current collecting stem. In particular, the embodiment shown in FIG. 2 comprises a current collecting stem (4), for instance a 1xc2xcxe2x80x3 (31.75 mm) stem with xc2xexe2x80x3 UNC female thread, fit for hosting a dowel screw (10) with the corresponding male thread. The electric contact between the anodic base (1) and the current collecting stem (4) is mostly assured by the tightening of the exposed copper part of such stem (4) to the copper current collecting bottom (11) of the anodic base (1). The simultaneous current flow from the copper bottom (11) to the dowel screw (10) through the thread of the tightening nut (12) is considered as negligible, both for the number of conduction interfaces and for the smaller section involved. The separation between the copper bottom (11) of the anodic base (1) and the anolyte is achieved, as above described, by means of an anodic liner (13) made of a titanium sheet, for instance a 1 mm thick sheet, perforated and activated in correspondence of the stems (4) which is also a fundamental integrating part of the anodic seal. The gasket (9) is typically a torus made of a hydrocarbon-based elastomer (for instance EPM or EPDM), pressed to the anodic liner (13) by means of a collar (14). The collar (14) is preferably made of a titanium-palladium alloy, to have a suitable resistance to crevice corrosion, and may have for instance a diameter of 50.0-50.8 mm and be welded at a distance of 4.7 mm from the bottom of the stem (4). The gasket (9) works therefore under predetermined deformation, which in the case of the aforementioned exemplary dimensions would be 3.7 mm in the lined zone. The typical starting thickness may be, for instance, of 6 mm, so as to achieve the typical compression ratio of 40%; even considering the whole contact region between toroidal rubber gasket (9) and anodic liner (13) as the effective seal bearing, it is evident how restricted is its width; for instance, for a collar (14) with a diameter of 50 mm in correspondence of a hole in the liner (13) with a diameter of 35 mm, the resulting width of the seal bearing zone is just 7.5 mm. The clamping load delivered by the dowel screw (10), which is normally made of brass or copper-nickel alloy, is limited by the mechanical resilience of the threaded portion of the current collecting stem (4); an indicative value, typical for xc2xexe2x80x3 UNC threaded pieces, is about 8 kg.m. The above described prior art suffers the following limitations:
the gasketing material (EPM, EPDM) has inadequate resistance to chlorine, in conjunction with a high surface exposed to the aggressive environment.
The use of composite gaskets with a PTFE protective coating is made impossible by the high ratio of seat to compressed thickness (about 2:1) and by the high compression ratio (40%).
On the other hand, the use of PTFE derived material, such as Gylon(copyright) (commercialised by Garlock, USA) or Permanite(trademark) Sigma (commercialised by TBA, United Kingdom), is impeded by the scarce compressibility and consequently by the need of using very high mechanic loads to assure the sealing.
The compression load is not well defined, since the gasket works at predetermined deformation, as above described.
The combination of these factors strongly limits the lifetime of the anodic sealing gaskets (9), hampering, as above disclosed, the whole economics of the operation of diaphragm cells. An attempt of solving the aforementioned problems is described in the Swedish Patent Application 97 020 79, and in the corresponding technology commercialised by Akzo Nobel under the trademark Tibac(trademark). Such finding consists in the direct welding of the collar (14) to the anodic liner (13), carried out by means of a laser. In this way, no polymeric material is used for sealing, with evident advantages in terms of reliability, as every polymeric gasketing material is to some extent subject to corrosion. By means of this technique, however, some undeniable drawbacks are introduced: as apparent, the anodes (2) aren""t anymore detachable from the anodic liner (13) and consequently from the base (1), with negative consequences both in terms of handling during assemblage and maintaining procedures, and of possibility of conveniently reactivating the anodes (2) once their catalytic coating becomes exhausted. Furthermore, the weld bead has a remarkable extent, and the risks of leakage due to local defects are therefore high. A second partial solution to the problem consists in the use of a gasket (9), provided with a lip (15) and shaped as shown in FIG. 3. The constructional principle contemplates the exposition to chlorine of a reduced elastomer surface. In this manner, the possibility of detaching the anodes (2) from the anodic base (1) is retained, guaranteeing at the same time an extended lifetime of the gasket (9) in view of its decreased exposition to corrosive agents. This finding has proved however to be not yet sufficient to assure an adequate reliability, being the gaskets (9) still subject to corrosion-induced leaks in an average longer, but still unpredictable time. Moreover, the construction tolerances, from which the state of compression of the lip (15), which is very thin, depends, become more critical; from the state of compression of the lip (15) depends, on its turn, the chemical resistance of the same. Finally, in this type of gasket the seal is entrusted to the inner shaped ring which, in case an infiltration occurs, is destined to a quick collapse as it is thinner than a traditional gasket.
Under a first aspect, it is an object of the present invention to provide a design of diaphragm electrolytic cell for the production of chlorine and alkali with improved reliability with respect to the state of the art, assuring a time of operation without maintenance or replacement of components being only limited by the lifetime of the NAD diaphragms.
Under another aspect, it is an object of the present invention to provide a seal system for anodes of diaphragm electrolytic cells for the production of chlorine and alkali preventing gasket corrosion phenomena for a minimum time of five years, at the same time retaining the possibility of detaching every single anode from the anodic liner.
Under another aspect, it is an object of the present invention to provide a seal system for anodes of diaphragm electrolytic cells applicable not only to cells of new construction, but also to cells designed and manufactured according to the prior art, eventually already in operation, allowing to prevent or overcome the occurrence of corrosion problems due to the fault of their seal system.
Under another aspect, it is a further object of this invention to provide a seal system for anodes of diaphragm electrolytic cells for the production of chlorine and alkali applicable to cells designed and manufactured according to the prior art which have already undergone pronounced corrosion phenomena, included the degradation of the anodic current collecting bottom (11).
A new configuration of hydraulic seal and electric contact between the anodic base (1) and the anodes (2) of a diaphragm electrolytic cell for the production of chlorine and alkali, allowing to entirely overcome the limitations of the prior art, is described herebelow. The constructional principle of the invention comprises a seal system based on an O-ring and a fixed mechanic spacer, and an electrical contact system based on the interposition of a conductive dimensionally adaptable intermediate layer between the anodic base (1) and the bottom of the current collecting stem (4). According to this new cell design, contrarily to what happens in the prior art, the component deformable upon cell clamping is an integral part of the electric contact, and not of the hydraulic seal. The innovative characteristics of the cell design of the invention are summarised in FIG. 4, and herebelow described. The hydraulic seal is based on an O-ring (16), instead than on the planar gasket (9), optionally provided with a lip (15), of the prior art. The O-ring (16) must have the following characteristics:
be manufactured starting from a chemically inert, and possibly elastic, construction material
have dimensions sufficient to compensate for the local irregularities
seat exclusively on the anodic liner (13)
have a low deformation load (for instance, substantially lower than a spirometallic seal).